Soft, fatigue resistant elastomer articles

ABSTRACT

A soft, fatigue resistant vulcanizate adapted for transmitting load between moving mechanical parts comprising (a) 100 parts by weight of crosslinked elastomer, (b) a fatigue life enhancing amount of substantially internally saturated, substantially linear polymer that (i) is made from monomers consisting essentially of isobutylene, (ii) is a strain crystallizable, elastic solid at 20° C. and (iii) has a viscosity average molecular weight (Flory) above about 1.3 million, (c) about 5-200 parts by weight particulate comprising carbon black, said elastomer being crosslinked with (d) curative in an amount sufficient to crosslink said elastomer, wherein said polymer of (b) is dispersed throughout said elastomer of (a) in a discrete microscopic phase.

This application is co continuation of application Ser. No. 192,782,filed Oct. 1, 1980 and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to (i) a method of preparing soft, fatigueresistant, elastomeric articles for transmitting load between movingmechanical parts through incorporation of certain isobutylene polymers,and (ii) articles prepared thereby. This invention, in preferredembodiments, relates to (i) a method of preparing soft, heat and fatigueresistant, elastomeric articles for transmitting load between movingmechanical parts comprising such isobutylene polymers and otheressential ingredients and (ii) articles prepared thereby.

Elastomeric articles of the type to which the invention applies havediverse applications. Vehicular applications include, for example,suspension components such as front or rear suspension bushings, enginemounts, etc. Elastomeric articles or parts in these applications receiveand transmit loads between mechanical components in relative motion withone another. The elastomeric articles, accordingly, require an optimalfatigue resistance as well as other desired properties.

It would be desirable in certain circumstances that the elastomericarticles or parts also be soft. For example, it would be desirable tohave a vehicular suspension bushing that had lower hardness as comparedto traditional suspension bushings. The lower hardness could alterfavorably vehicle ride characteristics in, for example, lighter, smallervehicles.

Physical alternation of an elastomeric part of this type so as to makeit softer may have concommittant drawbacks. For example, an increase inthe size of the elastomeric parts (e.g., bushing) generally reducesstiffness because, for a given deflection, the part will be under asmaller strain; an increase in size, however, is inconsistant with anobjective of lighter, smaller vehicles. Moreover, introduction of holesinto the elastomeric article also reduces stiffness; the holes, however,may introduce stress concentrations in the article.

An alternative is to formulate a soft elastomeric article; even here,however, there is difficulty. For example, traditional fatigue testsapply constant load or constant strain to an elastomeric article testsample. In a comparison between test samples of unequal moduli underconstant, repetitively applied loads, the softer test sample undergoesgreater strain; it, therefore, receives higher energy input. On theother hand, a constant strain test is more severe on the harder samplebecause an equal amount of strain in the harder sample requires greaterenergy input.

Test conditions that approximate equal energy inputs to hard and softsamples better compare basic fatigue life of the samples. Under suchconditions, it has been found that certain elastomeric articlesformulated to be soft do not have comparable fatigue life to harderproduction counterparts.

An exception to usually diminished fatigue life of soft elastomericarticles of the above described type now has been discovered. Certainpolymers of isobutylene have been found, at certain levels, not only topermit softer elastomeric articles or parts but, also, to give articleswith desired fatigue life.

This invention may be practiced fully without any appreciation of thetheoretical principles underlying such a discovery. Indeed, thisinvention should not be limited by any characterization of suchprinciples. It is believed, however, that the isobutylene polymercrystallizes during strain of the article. The strain induced crystalsprevent or reduce crack or other flaw propagation. Prevention orreduction of flaw propagation enhances fatigue life. At the same time,the relative inertness of the isobutylene polymer to crosslinking allowsit to soften the elastomeric article.

It is an object of this invention to provide soft elastomeric articlesfor transmitting loads between parts suitable for fatigue producingapplications.

An additional, but independent, aspect of this invention is provision ofsoft, fatigue and heat resistant elastomers. In smaller vehicles, forinstance, elastomeric articles may be closer to engine or engine relatedcomponents. Thus, the elastomeric part may be exposed to hightemperatures for prolonged periods.

In accordance with this additional aspect of this invention, it has beendiscovered that certain isobutylene polymers may be formulated withnatural or synthetic polyisoprene rubber and elastomeric polybutadieneas well as other ingredients to yield a soft elastomeric article thatmeets or exceeds many commercially available goods in certain fatigueand heat resistant properties.

Accordingly, it is an additional, independent object of this inventionto provide soft, heat and fatigue resistant, elastomeric articles fortransmitting loads between mechanical parts.

This invention accomplishes these and other objects as will be apparentto those in the art from the disclosure hereinafter set forth.

BRIEF DESCRIPTION OF THE INVENTION

This invention relates to a method of prearing a soft, fatigueresistant, elastomeric article for transmitting loads between movingmechanical parts. The method comprises:

A. Providing an intimately admixed dispersion comprising (a) 100 partsby weight of crosslinkable elastomer; (b) a fatigue enhancing amount ofa substantially internally saturated, substantially linear polymer that(i) is made from monomers consisting essentially of isobutylene, (ii) isa strain crystallizable elastic solid at 20° C. and (iii) has aviscosity average molecular weight (Flory) above about 1.3 million; (c)about 5-200 parts by weight particulate and (d) a curative in an amountsufficient to crosslink the elastomer; and

B. Maintaining the dispersion at a temperature sufficient to crosslinkthe elastomer.

The elastomeric articles of this invention include soft, fatigueresistant, vulcanizates which comprise 100 parts by weight crosslinkedelastomer having dispersed throughout in a discrete microscopic phaseabout 10-75 parts by weight of substantially internally uncrosslinked,substantially linear polymer of aforementioned characteristics. Thevulcanizates have application as, for example, suspension bushingshaving a Shore A hardness below about 60, e.g., 40-50.

A preferred embodiment of this invention is a method of preparing asoft, heat and fatigue resistant, elastomeric article for transmittingloads between moving mechanical parts. This method comprises: A.providing an intimately admixed dispersion comprising (a) 100 parts byweight of crosslinkable elastomer consisting essentially of (i) naturalor synthetic polyisoprene rubber and (ii) elastomeric polybutadiene madefrom monomers consisting essentially of butadiene at a weight ratio of(i) to (ii) of about 1:10 to 10:1; (b) about 5-55 parts by weight of theaforementioned substantially internally saturated, substantially linearpolymer; (c) curative comprising a curing agent selected from the groupconsisting of (i) a sufficient amount of sulfur to provide an efficientor semi-efficient vulcanization of the crosslinkable elastomer, (ii)isocyanate or blocked isocyanate in an amount sufficient to crosslinkthe crosslinkable elastomer and (iii) isocyanate or blocked isocyanateand sulfur in amounts sufficient to crosslink the crosslinkableelastomer and (d) about 5-200 parts by weight reinforcing particulate;and B. maintaining the dispersion at a temperature sufficient to curethe elastomer.

Additionally, this invention relates to a soft, fatigue resistant carbonblack reinforced vulcanizate adapted for transmitting a load betweenmoving mechanical parts comprising 100 parts by weight crosslinkedelastomer having about 15-35 parts by weight of substantially internallyuncrosslinked, substantially linear polymer that (i) is made frommonomers consisting essentially of isobutylene, (ii) is an elastic solidat 20° C. and (iii) has a viscosity average molecular weight (Flory)above about 1.3 million in a disperse phase consisting essentially ofparticles of the polymer below about 2 micrometers in diameter.

More particularly and preferably, this invention relates to a soft, heatand fatigue resistant vulcanizate adapted for transmitting a loadbetween moving mechanical parts comprising (a) 100 parts by weightcrosslinked elastomer consisting essentially of (i) natural or syntheticpolyisoprene rubber and (ii) elastomeric polybutadiene made frommonomers consisting essentially of butadiene at a weight ratio of (i) to(ii) of about 1:10-10:1; (b) about 15-55 parts by weight ofsubstantially internally uncrosslinked, substantially linear polymerthat (i) is made from monomers consisting essentially of isobutylene,(ii) is a strain crystallizable, elastic solid at 20° C. and (iii) has aviscosity average molecular weight (Flory) above about 1.3 million, theelastomer being crosslinked with (c) a curative comprising a curingagent selected from the group consisting of (i) sulfur in an amountsufficient to provide an efficient or semi-efficient vulcanization ofthe elastomer (ii) isocyanate or blocked isocyanate in an amountsufficient to crosslink the elastomer and (iii) isocyanate or blockedisocyanate and sulfur in amounts sufficient to crosslink the elastomerand (d) about 5-200 parts by weight particulate wherein the polymer of(b) is dispersed throughout the elastomer of (a) in a discretemicroscopic phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates Example 1 data in which polyisobutyleneloading is plotted against fatigue life (constant energy) and againsttear resistance.

FIGS. 2 and 3 graphically illustrate fatigue life over a range of inputenergies for a production compound, R-1, and a compound of thisinvention, R-1415.

FIG. 4 graphically illustrates dynamic mechanical properties of aproduction compound R-1 and a compound of this invention.

FIG. 5 graphically illustrates tan delta over a range of temperaturesfor the compounds in FIG. 4.

FIGS. 6-9 graphically illustrate properties of a production compound andcompounds made in accordance with this invention.

FIG. 10 graphically illustrates dynamic properties of a productioncompound and compounds of the invention versus temperature.

FIG. 11 graphically illustrates dynamic properties of a productioncompound and a compound, R-1583, of this invention.

FIGS. 12 and 13 graphically illustrates tan delta for a productioncompound and compounds R-1583 and R-1587 (FIG. 12) and R-1589 and R-1590(FIG. 13) of this invention.

FIG. 14 is a photomicrograph of unstained R-1415, a compound of thisinvention at 46,000x.

FIG. 15 is photomicrograph of R-1415, stained to color double bonds. Thewhite particles are polyisobutylene. The photomicrograph is at 46,000x.

DETAILED DESCRIPTION OF THE INVENTION

The crosslinkable elastomer employed in preparation of articles of thisinvention preferably comprises natural or synthetic polyisoprene rubber,more preferably natural rubber. Natural rubber strain crystallizes and,accordingly, is exceptionally suited for fatigue producing applications.

However, other crosslinkable elastomers, particularly straincrystallizable elastomers, may replace, in part or total, the natural orsynthetic polyisoprene rubber. For example, polychloroprene is straincrystallizable, although less than natural rubber, and may be softened.Still other crosslinkable elastomers include chlorobutyl or bromobutylrubber as well as amorphous elastomers, e.g., butadiene-styrene rubberor nitrile elastomers. These elastomers may likewise be softened by theisobutylene polymers of this invention. Combination (blends) of suchelastomers may also be employed as the elastomer component of thecomposition of this invention. Selection of the particular elastomercomponent optimally suited for use in a particular application would bewell within the skill of those in the art. For example, the addition ofbromobutyl rubber to natural polyisoprene rubber is known to improvedamping properties of the elastomers.

In one particularly preferred embodiment of the invention composition, asoft, fatigue resistant vulcanizate with improved damping properties isobtained when the elastomer component comprises natural or syntheticpolyisoprene rubber and bromobutyl rubber. In this embodiment, theelastomer preferably comprises natural or synthetic polyisoprene rubberand bromobutyl rubber in a weight ratio of between about 100:1 and about1:1, most preferably between about 7:3 and about 1:1. Preferably, asemi-EV sulfur cure system is employed in curing the elastomer of thisembodiment.

In fatigue producing applications where high heat resistance also issought, the articles of this invention have a crosslinkable elastomerthat, in addition to natural or synthetic polyisoprene rubber,preferably further comprises elastomeric polybutadiene and certain otheringredients.

Elastomeric polybutadiene is commercially available; it is made byeither solution or emulsion polymerization. Preferred polybutadiene ismade from monomers consisting essentially of butadiene. An especiallypreferred polybutadiene has a cis content greater than 50% by weight,more preferably a cis content at least about 98% by weight.

As mentioned, elastomeric polybutadiene and natural rubber are togetherin preferred embodiments which offer an optimally heat resistant as wellas soft, fatigue resistant elastomeric article. In these preferredembodiments, the curatives are preferably of certain character, as willbe mentioned hereinafter.

The elastomeric articles of this invention have certain straincrystallizable, isobutylene polymers that soften the articles in fatigueenhancing amounts. Preferably, about 10-75 parts by weight per 100 partsby weight of the aforementioned crosslinkable elastomer in the articlescomprises this polymer. In certain formulations, it has been found thatmore desirable fatigue resistant properties occur, particularly withreinforcing filler, at between about 15-55, more preferably betweenabout 15-35 parts by weight of the polymer per 100 parts of thecrosslinkable elastomer. For polybutadiene containing articles of thisinvention, an especially preferred range of isobutylene polymer isbetween about 26-34 parts per 100 parts of crosslinkable elastomer.

The polymer which softens as well as maintains or improved fatigueresistance of the elastomeric article is a substantially internallysaturated, substantially linear polymer made from monomers consistingessentially of isobutylene, is a strain crystallizable elastic solid atroom temperature, and has a viscosity average molecular weight (Flory)above about 1.3 million, more preferably above about 1.5 million, and anespecially preferred range of between about 1.8 and about 2.5 million.The polymers of isobutylene may be terminally unsaturated. They arecommercially available polymers. For example, Exxon markets severalgrades of polyisobutylene polymer as Vistanex polyisobutylene. Of theseterminally unsaturated polymers, those with viscosity average molecularweights (Flory) between about 2.0 and about 2.2 million are especiallypreferred at the above indicated preferred amounts.

The elastomeric articles of this invention also preferably compriseorganic or inorganic particulate. Examples of preferred organic andinorganic particulate include carbon blacks, zinc oxide, fine particlecalcium carbonate, silicas and silicates. Preferably, the particulatecomprises reinforcing particulate such as carbon black. Othernonreinforcing particulates may also be suitably employed to modify orextend the elastomeric articles.

Particulate levels range desirably between about 5-200 parts by weightper 100 parts by weight crosslinkable elastomer. Preferred carbon blacklevels range between about 5-200 parts by weight, more preferably about20-80 parts by weight. The level of carbon black has been determined toaffect hardness; elastomeric articles with a Shore A hardness belowabout 60, normally have less than 200 parts by weight carbon black,preferably up to about 75 parts by weight. Preferred carbon blackscomprise carbon blacks having an average particle size between about20-100 millimicrons. Also, preferred carbon blacks have a dibutylphthalate absorption (cc/100 g) of about 70-120.

Combinations of reinforcing particulates may be suitably employed. Forexample, the reinforcing filler may comprise carbon black and finelydivided silica at equal amounts by weight.

As long as the curative for the elastomer does not crosslink thesubstantially saturated portion of the polymer made from isobutylene,there is little limitation to the type of curative that may be suitablyemployed. For example, conventional sulfur vulcanization (i.e.,relatively high levels of sulfur to accelerator) may be used to preparesoft vulcanizates of excellent fatigue resistance in accordance withthis invention.

For soft, fatigue and heat resistant elastomeric articles, the curativepreferably comprises a curing agent selected from the group consistingof (i) sulfur (elemental or otherwise), (ii) isocyanate or blockedisocyanate and (iii) isocyanate or blocked isocyanate and sulfur(elemental or otherwise).

With respect to sulfur containing curatives (i), the sulfur ispreferably used in an amount sufficient to provide efficient orsemi-efficient vulcanization of the crosslinkable elastomer, morepreferably, semi-efficient vulcanization. The isocyanate or blockedisocyanate may be used at conventional levels, e.g., an amountsufficient to crosslink the crosslinkable elastomer. The isocyanate orblocked isocyanate and sulfur, more preferably employed in thisinvention, are used desirably so that the isocyanate or blockedisocyanate is predominant, as hereinafter described in greater detail.

Examples of specific effecient vulcanization (EV) and semi-efficientvulcanization (Semi-EV) curatives appear in, for example, NOVOR,Natural/Synthetic Rubber Crosslinkers) Bulletin No. 8006A of HughsonChemicals, p. 9, 2-26. Another example of EV curatives appears inBritish Pat. No. 1,255,355; also in NR TECHNOLOGY, Rubber DevelopmentSupplement, 1972, No. 8.

Of the sulfur containing curatives (i) (sulfur being used herein torefer elemental sulfur or sulfur donor unless otherwise stated to thecontrary) Semi-EV curatives are preferred. Such Semi-EV curativescomprise an intermediate sulfur to accelerator ratio, e.g., 0.6-2.4.Preferred semi-EV curatives comprise sulfur donors and elemental sulfur.

Examples of accelerators for use in sulfur containing curatives include(a) thiazolesulfenamides such as benzothiazolesulfamides; (b)thiocarbamylsulfenamides; (c) phosphinothioylaminosulfides and (d)thiozyldisulfides. Examples of sulfur donors (also known as vulcanizingagents) are (a) dithioamines; (b) (iminodithio) thiazoles; (c)aminothiocarbamyldisulfides and (d) thiuram disulfides. Other examplesof accelerators and sulfur donors are known to the art. A list ofcommercially available accelerators and sulfur donors appears in, forexample, Rubber Worlds Blue Book entitled, Mat., Comp. Inc. andMachinery For the Rubber Industry (1980) by Rubber/Automotive Divisionof Hartman Communications, Inc., a Subsidiary of Bill Communicaions,Inc. (633 Third Ave., New York, N.Y. 10017). Still another list ofsuitably employed accelerators and sulfur donors (vulcanizing agents)appear in Rubber Chemistry and Technology, 53, July-August, 1980, No. 3in the chapter entitled "S-N Compounds As Delayed Action Chemicals inVulcanization."

Another preferred curative comprises isocyanate or blocked isocyanate.This curative is well known. Examples appear in U.S. Pat. Nos.3,904,592; 3,882,089; 3,775,441; 3,645,980; as well as Baker, C. S. L.et al, Urethane Crosslinking of Natural Rubber, International RubberConference, P. G2 through G2-8 (1972).

Preferred isocyanate or blocked isocyanate containing curatives comprisea reaction product of nitrosophenol and di or polyisocyanate. A specificexample is a urethane product of 2,4-toluene diisocyanate dimer and4-nitroso-2,6-xylenol. Commercially available curing agents comprising aproduct of this type are are NOVOR TM 913, 920 and 924 available fromDurham Chemicals Ltd., Birtley, Co., Durham, England.

Especially preferred curatives, however, for soft, heat and fatigueresistant elastomeric articles of this invention comprise a curing agentwhich is a combination of sulfur and isocyanate or blocked isocyanate. Acombination of sulfur and isocyanate or blocked isocyanate isillustrated in U.S. Ser. No. 796,114 filed May 11, 1977, in the name ofMarano (now abandoned) which is hereby herein expressly incorporated byreference. In the combined isocyanate or blocked isocyanate and sulfursystems of U.S. Ser. No. 796,114, the sulfur is used at rubber solublelevels. Additionally, the sulfur accelerator is a catalyst for theurethane.

A particularly preferred isocyanate or blocked isocyanate combinationcomprises (per 100 parts of elastomer) combinations of isocyanate orblocked isocyanate and sulfur used such as follows in Tables I or II:

                  TABLE I                                                         ______________________________________                                               NOVOR 924                                                                              ZDAC.sup.1 Sulfur  S.sup.2                                    ______________________________________                                        90/10    6.03       1.8        0.25  0.05                                     80/20    5.36       1.6        0.5   0.1                                      70/30    4.69       1.4        0.75  0.15                                     60/40    4.02       1.2        1.0   0.2                                      50/50    3.35       1.0        1.25  0.25                                     ______________________________________                                         .sup.1 Zinc diloweralkyldithiocarbamate accelerator such as zinc              dimethyldithiocarbamate.                                                      .sup.2 Sulfenamide accelerator such as                                        N--tbutyl-2-benzothiazolesulfenamide.                                    

                  TABLE II                                                        ______________________________________                                               NOVOR 924                                                                              TMTM.sup.3 Sulfur  S.sup.4                                    ______________________________________                                        90/10    4.8        1.4        0.2   0.04                                     80/20    4.2        1.3        0.4   0.08                                     70/30    3.8        1.2        0.6   0.12                                     60/40    3.2        1.1        0.8   0.16                                     50/50    2.7        1.0        1.0   0.20                                     ______________________________________                                         .sup.3 Tetramethylthiuram monosulfide accelerator.                            .sup.4 Sulfenamide accelerator such as                                        N--tbutyl-2-benzothiazolesulfenamide.                                    

In Tables I and II, the NOVOR 924 may be replaced in part e.g., 50% byweight by toluene diisocyanate dimer.

A particularly preferred sulfur-urethane range is between the 90/10 and70/30, listed above in Tables I and II, NOVOR 924 to sulfur.

Besides elastomer, isobutylene polymer, reinforcing particulate, andcuratives for the elastomer, the elastomeric articles of the inventionmay desirably also include still other ingredients. Examples of suchingredients are antioxidants (e.g., polymerized quinolines, hinderedamines, phenols), dessicants (e.g., calcium oxide), process oils, cureinhibitors or modifiers and the like known in the art.

The elastomeric articles of this invention may be compounded usingconventional equipment. It is important, however, to intimately admixthe isobutylene polymer, the elastomer and other ingredients. This maybe achieved, for example, on two roll rubber mills, Banbury mixers andmill and mixer combinations. The elastomer component preferably isadmixed with the isobutylene polymer in a Banbury mixer prior toincorporation of the curing system. Particulates are normally admixed inthe Banbury before curative addition. Thereafter, the curing agents areadded, on a mill or in the Banbury. The curatives are preferably addedat a temperature below about 120° C., e.g., 60°-80° C.

Once compounded, the elastomeric article may be cured at any convenienttemperature; a preferred range for curing, however, is between about120°-190° C., more preferably 150°-180° C. Cure time is preferably atleast about 80%, more preferably at least about 90% of the time to reachmaximum torque development on, for example, a Monsanto oscillating discrheometer (ASTM-D2084-71T). Temperatures above about 160° C. during cureenhance physical properties including fatigue life of elastomericarticles of this invention. Enhanced physical properties at higher curetemperatures indicate that morphology plays a role in preparation ofsoft, fatigue resistant elastomeric articles of this invention.

Applications for elastomeric articles of this invention are diverse, aspreviously mentioned. For automotive suspension bushings, theelastomeric articles of this invention preferably are compounded to haveconstant energy fatigue life (see examples for description) of at leastabout 60 kilocycles, e.g., 75 kilocycles. Shore A hardness (ASTM D2240)below about 55, e.g., 40-50; and compression set (D395 (Method B), 22hours at 150° C.) below about 50%, more preferably below about 35%.

The following examples illustrate embodiments of this invention; theinvention, of course, is not limited to these embodiments, but, rather,embodiments within the scope of claims hereinafter presented.

EXAMPLE 1

The natural rubber (NR) used in this example was SMR-5L. Thepolyisobutylene (PIB) was obtained from Exxon Chemical Company. The PIB(Vistanex MM L-80 or L-140) had respective Flory viscosity averagemolecular weights of 1×10⁶ and 2.1×10⁶, according to "VistonexPolyisobutylene Properties and Applications", Exxon Chemical Company,1974. Compounds R-1 and R-2 (unknown formulations) were obtained fromsuppliers of front automotive bushing compounds. They are believedrepresentative of commercially used production compounds. Ingredientslisted in Table 1A below were mixed in a Banbury mixer (Model BR) usinga six minute mixing schedule (ASTM D15-72) to make elastomeric goods ofthis invention. The curatives were added on a cooled 200×400 mm two rollmill. Cure properties were determined on an oscillating disk rheometer.

Sample sheets (150×150×2 mm) were molded according to ASTM D3182;compression set buttons (28 mm diameter and 13 mm thickness) were madeaccording to ASTM D395. Specimens were cured to 95% of optimum cure asdetermined using the oscillating disk rheometer. Tensile and tearspecimens were die cut from the sheets with a punch press. Fatiguespecimens (rings) were cut from the sheets using a two bladed flycutter. The rings had an i.d. of about 26 mm and wall thickness ofapproximately 0.7 mm.

                  TABLE 1A                                                        ______________________________________                                                   R-1457                                                                              R-1414  R-1445  R-1415                                                                              R-1444                                 ______________________________________                                        SMR-5L       100     100     100   100   100                                  Vistanex L-140.sup.1                                                                       --      --      10    20    40                                   Vistanex L-80.sup.2                                                                        20      --      --    --    --                                   N-765.sup.3  34      34      34    34    34                                   Zinc Oxide   5       5       5     5     5                                    Stearic Acid 2       2       2     2     2                                    Agerite Resin D.sup.4                                                                      2       2       2     2     2                                    Santoflex 13.sup.5                                                                         1       1       1     1     1                                    Dutrex 419.sup.6                                                                           5       5       5     5     5                                    Durax.sup.7  0.5     0.5     0.5   0.5   0.5                                  Sulfur       2.5     2.5     2.5   2.5   2.5                                  Tensile (MPa)                                                                              20.6    25.1    21.3  20.6  16.4                                 Elong. (%)   620     600     630   620   630                                  Tear (KN/m)  47.7    63.5    60.9  47.7  35.4                                 Hardness (Shore A)                                                                         42      44      43    42    40                                   ______________________________________                                         .sup.1 polyisobutylene having a viscosity average molecular weight (Flory     of about 2.1 million available from Exxon Chemical.                           .sup.2 polyisobutylene having a viscosity average molecular weight (Flory     of about 1.0 million available from Exxon Chemical.                           .sup.3 Carbon black.                                                          .sup.4 Polymerized 1,2 dihydro 2,2,4trimethylquinoline.                       .sup.5 N--(1,3dimethylbutyl)-N--phenylp-phenylenediamine.                     .sup.6 Process oil.                                                           .sup.7 N--cyclohexyl2-benzothiazole-sulfenamide.                         

Test Methods

Tensile strength and elongation at break were determined according toASTM D412 (die C) and tear strength according to ASTM D624 (razor notchdie B). Heat aging of samples was carried out in a ventilated aircirculating oven for two hours at 150° C.

Compression set testing was done according to ASTM D395 (method B) oncompression set buttons. The test conditions were 22 hours at 125° C.under 25% compression in a ventilated, air circulating oven.

Hardness of the vulcanizate was measured according to ASTM D2240 after a30 second relaxation using a Shore A durometer.

Dynamic mechanical properties were determined in compression usingcompression set buttons on an Instron 1350 servohydraulic test machine.The specimen was confined between two parallel plates with 150 gritsandpaper on the plates to prevent slippage. To simulate the strainconditions typically seen by a suspension mount, the buttons wereprestrained to 30% compression and allowed to relax under load 15minutes at the test temperature. The sinusoidal strain of ±1% wassuperimposed at a frequency of 10 Hz. Test temperatures ranged from -40°to 100° C. and the samples were soaked at least 1/2 hour at the testtemperature before applying the prestrain. Prior to any testing, eachbutton was preconditioned by applying a 40% compressive strain. Theelastic and storage moduli were then obtained from the Lissajou figuresobtained by plotting load versus strain.

Fatigue life measurements were made using the Instron servohydraulictester and ring specimens. The rings were suspended from two spindles,one attached to the load cell and the other to the hydraulic actuator.The spindles were lubricated with glycerin to insure that abrasion didnot contribute to the failure of the rings. The test frequency was 3Hertz; failure was defined as breaking of the ring. The strain energy ofany particular cycle was determined from the stress-strain curverecorded on an X-Y plotter.

The ring fatigue test described above approximates an equal energy inputby using a constant strain test. The test strain in this test was chosenso that the input energy at the beginning of the test was the same foreach material. The energy input to the specimen decreased during thetest. This resulted primarily from stress softening of the elastomer; asmall amount resulted from crack formation and propagation. The behaviorwas similar among the materials tested. A strain energy input of 1.4mJ/mm³ was chosen as the test condition for surveying the effect ofmodulus on fatigue life. Under these conditions, a commerciallyavailable natural rubber compound of 60 Shore A durometer hardness has afatigue life of 60 Kc.

FIG. 1 shows the effect of various amounts of polyisobutylene (compoundsother than 1457) on fatigue and tear strength. Tear strengthmonotonically decreases with an increase in polyisobutylene; thisbehavior is characteristic of several other physical propertiesincluding tensile stength and hardness.

A comparison of compound 1415 with compound 1457 in Table 1B shows thatthe higher molecular weight polyisobutylene gives better fatigue life.

                  TABLE 1B                                                        ______________________________________                                                                           Fatigue                                    Compound Molecular Wgt.                                                                            Hardness (Shore A)                                                                          Life (Kc)                                  ______________________________________                                        1415     2.1 × 10.sup.6                                                                      42            65                                         1457     1.0 × 10.sup.6                                                                      40            31                                         ______________________________________                                    

Table 1C below shows the effect of cure temperature on hardness andfatigue.

                  TABLE 1C                                                        ______________________________________                                                              Hardness                                                Compound Cure Temp. (°C.)                                                                    (Shore A)   Fatigue (Kc)                                ______________________________________                                        1415     130          46          74                                          1415     150          42          65                                          1415     170          35          61                                          ______________________________________                                    

Fatigue life is dependent upon the input energy and the amount of straininduced crystallization which occurs in the elastomer. A relationshipbetween fatigue life, input energy, flaw size and extent ofcrystallization is (Payne and Whittaker, Rubber Chem. Technol. 45, 1043(1972)): ##EQU1## where N is the fatigue life, G is the cut growthconstant, k a varying function of strain, W the strain energy asmeasured from the retraction stress-strain curve, Co is the initial flawsize, and n is a constant which depends upon the extent of straincrystallization. n has values of 2.0 for crystallizing rubbers like NR.A comparison of the fatigue life behavior of a production compound (R-1)and a NR/PIB blend was made over a range of input energies from 0.7 to2.9 mJ/mm³. The results are shown in FIGS. 2 and 3. (FIG. 2 shows theproduction compound results; FIG. 3 shows 1415 compound results.) From alinear regression analysis of the data, the slopes (-n) are both thesame, +2.3.

To further characterize the NR/PIB blend, a study of the dynamicmechanical properties, thermal and oxidative stability and compressionset were undertaken. A comparison of the dynamic mechanical propertiesof a production compound (R-2) and the NR/PIB blend as a function oftemperature are shown in FIGS. 4 and 5. The plot (FIG. 4) of elasticmodulus (E') shows that the blend is dynamically softer than theproduction compound over the entire temperature range. The plots inFIGS. 4 and 5 also show that the blend exhibits a more pronouncedtemperature sensitivity than the production compound. (The squares aredata points for the production compound, the circles are data points forcompound R-1415.)

Table 1D summarizes the data on the compression set and thermal andoxidative stability of the blend and the two production compounds.

                  TABLE 1D                                                        ______________________________________                                        HEAT AGED PHYSICAL PROPERTIES AND                                             COMPRESSION SET DATA FOR TWO PRODUCTION                                       COMPOUNDS (R-1 AND R-2) AND THE NR/PIB BLEND                                  Tensile        Tear           Compression                                     Strength, MPa  Strength KN/m  Set, %                                          Un-     Heat Aged 2                                                                              Un-     Heat Aged 2                                                                            22 hrs                                    aged    hrs @ 150° C.                                                                     aged    hrs @ 150° C.                                                                   @ 125° C.                          ______________________________________                                        R-1  27.2   9.2        85.0  52.5     38.7                                    R-2  24.1   6.8        77.9  47.3     50.8                                    1415 16.9   6.0        40.2  22.6     71.4                                    ______________________________________                                    

Electron microscopic analysis of R-1415 shows that the isobutylenepolymer is a discrete phase of particles below 2 micrometers in diameteras is seen in FIG. 15; FIG. 14 is a section as in FIG. 15, but withoutany stain. Both are at 46,000×.

EXAMPLE 2

The natural rubber used in this Example was SMR-5L. The polyisobutylene(PIB) was Vistanex MM L-140, described previously. The polybutadiene(BR) was Goodyear Tire and Rubber Company Budene 1207; it had acis-content of 98% by weight. The natural rubber compound R-1 was afront suspension bushing from a commercial manufacturer.

Ingredients listed in Table A were mixed in a Banbury mixer (BR) using asix minute (ASTM D15-72) mixing schedule. Th cure ingredients were addedon a cooled 200×400 mm two roll mill. Vulcanization parameters weredetermined by means of an oscillating disk rheometer. The compounds weremolded and cured to 95% optimum cure at 150° C. or 170° C.

                                      TABLE 2A                                    __________________________________________________________________________               R-1459                                                                            R-1466                                                                            R-1540                                                                            R-1541                                                                            R-1570                                                                            R-1571                                                                            R-1572                                                                            R-1573                                                                            R-1583                                                                            R-1587                                                                            R-1588                                                                            R-1589                                                                            R-1590             __________________________________________________________________________    Natural Rubber                                                                           100 100 100 100 100 100 100 100 100 75  50  50  50                 Polyisobutylene                                                                          20  20  20  20  20  20  20  20  30  30  30  30  30                 Polybutadiene                                                                            --  --  --  --  --  --  --  --  --  25  50  50  50                 Carbon Black                                                                             50  40  50  50  25  40  40  40  50  50  50  45  45                 Hydrated Silica                                                                          --  --  --  --  10  --  --  --  --  --  --  --  --                 Zinc Oxide 5   5   5   5   5   5   5   5   5   5   5   5   5                  Stearic Acid                                                                             2   2   2   2   --  2   2   2   2   2   2   2   2                  Polymerized 1,2-di-                                                                      2   2   2   2   2   2   2   2   2   2   2   2   2                  hydro-2,2,4-trimethyl-                                                        quinoline                                                                     N--(1,3-dimethyl-                                                                        1   1   1   1   2   2   2   2   1   1   1   1   1                  butyl)-N--phenyl-p-                                                           phenylenediamine                                                              Process Oil                                                                              5   5   5   5   --  5   5   5   5   5   5   5   5                  (Dutrex 419)                                                                  2-Mercaptobenz-                                                                          --  --  --  --  1   --  --  --  --  --  --  --  --                 imidazole                                                                     Zinc 2-ethylhexoate                                                                      --  --  --  --  2   --  --  --  --  --  --  --  --                 Sulfur     2.50                                                                              0.40                                                                              0.40                                                                              0.60                                                                              0.40                                                                              0.40                                                                              0.60                                                                              1.45                                                                              0.40                                                                              0.40                                                                              0.40                                                                              0.40                                                                              0.60               N--cyclohexyl-2-                                                                         0.50                                                                              --  --  --  --  --  --  --  --  --  --  --  --                 benzothiazole-                                                                sulfenamide                                                                   N--t-butyl-2-benzo-                                                                      --  0.08                                                                              0.08                                                                              --  0.08                                                                              0.08                                                                              --  0.25                                                                              0.80                                                                              0.80                                                                              0.80                                                                              0.80                                                                              --                 thiazole-sulfenamide                                                          N--oxydiethylene-2-                                                                      --  --  --  0.75                                                                              --  --  0.75                                                                              --  --  --  --  --  0.70               benzothiazole-                                                                sulfenamide                                                                   Tetramethylthiuram                                                                       --  0.90                                                                              0.90                                                                              --  0.90                                                                              0.90                                                                              --  0.45                                                                              0.90                                                                              0.90                                                                              0.90                                                                              0.90                                                                              --                 monosulfide                                                                   N--oxydiethylenethio-                                                                    --  --  --  1.50                                                                              --  --  1.50                                                                              --  --  --  --  --  1.40               carbamyl-N'--oxy-                                                             diethylenesulfenamide                                                         Urethane Crosslinker                                                                     --  4.20                                                                              4.20                                                                              --  4.20                                                                              4.20                                                                              --  2.10                                                                              4.20                                                                              4.20                                                                              4.20                                                                              4.20                                                                              --                 (NOVOR 924)                                                                   __________________________________________________________________________

Sample sheets (150×150×2 mm) were molded according to ASTM D3182 andcompression set buttons (28 mm diameter by 13 mm thick) according toASTM D 395. Tensile, tear and ring specimens for fatigue testing werecut from the cured sheets using a die and punch press. A smaller ringspecimen used for obtaining the energy of the fatigue test was cut fromthe cured sheets using a two bladed fly cutter.

Tensile strength and elongation were determined at room temperatureaccording to ASTM D412 (die C) and tear strength according to ASTM D624(die B). Testing was done at 500 mm per minute on an electromechanicaltester.

Compression set testing was done according to ASTM D395 (method B). Thetest specimens were under 25% compression for 22 hours at 125° C. in aventilated, air circulating oven.

Hardness was measured according to ASTM D2240 on the unaged and heataged samples using a Shore A durometer. A 30 second relaxation wasallowed before the final reading.

Fatigue measurements were made using a Wallace-MRPRA fatigue tester andring specimens (O.D. 52.6 mm, I.D. 44.6 mm). The rings were lubricatedwith glycerine to prevent abrasion and mounted on four moving pulleys,two on the moving frame and two on the stationary frame. The rings werecyclically deformed in tension to a strain amplitude of either 162.5 or175% at 5 Hz until failure. Failure is defined as the breaking of thering. The reported fatigue life for each compound is the average of theresults from 12 specimens.

The strain energy of the fatigue test was determined using the Instron1350 servohydraulic test machine and ring specimens cut from ASTM testsheets. The rings were lubricated with glycerin and suspended from twospindles, one attached to the hydraulic actuator and the other to theload cell. The strain energy was then calculated using the stress-straincurve recorded on an X-Y plotter. A strain energy input of 1.4 mJ/mm³was used as the test condition for evaluating the fatigue life.

Dynamic mechanical properties were determined in compression usingcompression set buttons on an Instron servohydraulic test machine. Thesamples were confined between two parallel plates with 150 gritsandpaper attached to the plates to prevent specimen slippage. A 40%compressive strain was applied to the specimens several times tominimize the ribber-filler effects (Mullins effect). A 30% staticcompressive strain was applied and then the specimen was allowed torelax under load for 15 minutes at the test temperature. A sinusoidalstrain of ±1% was superimposed upon the static strain at a frequency of10 Hz. The samples were tested over a temperature range of -40° C. to100° C. The specimens were preconditioned at the test temperature for 30minutes prior to applying the static prestrain. The elastic and storagemoduli were determined from the Lissajou figures obtained by plottingthe load versus strain.

Heat aging of the fatigue samples was done according to ASTM D573-67 fortwo hours at 150° C. in a ventilated, air circulating oven.

Table 2B shows results obtained from testing the production compound R-1and R-1459. As can be seen, R-1459 is softer and has superior fatigue.R-1459, however, has less thermal stability than R-1.

Table 2C shows results of incorporating polyisobutylene into varioussulfur, urethane and mixed sulfur and urethane cured compounds.Compounds 1587, 1588, 1589 and 1590 have added polybutadiene.

                  TABLE 2B                                                        ______________________________________                                        PROPERTIES OF R-1459 AND THE                                                  PRODUCTION COMPOUND (R-1)                                                                         R-1459                                                                              R-1                                                 ______________________________________                                        Fatigue (Kc)          105     62                                              Hardness (Shore A)    44      66                                              Compression Set (%)   70      39                                              Heat Aged Tensil (MPa)                                                                              6.6     8.5                                             Heat Aged Elongation (%)                                                                            385     270                                             Heat Aged Tear Strength (KN/m)                                                                      21.3    42.5                                            ______________________________________                                    

                  TABLE 2C                                                        ______________________________________                                                                      Compression                                            Fatigue (Kc)                                                                           Hardness (Shore A)                                                                          Set (%)                                         ______________________________________                                        R-1540   33         47            31                                          R-1540   35         47            29                                          R-1543   38         51            --                                          R-1466.sup.(170).spsp.1                                                                44         41            33                                          R-1570   41         40            42                                          R-1571   40         41            55                                          R-1572   43         46            59                                          R-1573   48         45            62                                          R-1459   105        44            70                                          R-1587   63         46            33                                          R-1588   100        47            28                                          R-1589   97         45            27                                          R-1590   80         45            --                                          R-1583   53         44            31                                          ______________________________________                                         .sup.1 Cured at 170° C.                                           

FIGS. 6-9 show graphically a comparison between the production compoundand compounds with varying amounts of polybutadiene (BR). Resultsobtained on heat aging appear as broken lines; results obtained withoutheat aging appear as solid lines.

FIG. 10 shows dynamic mechanical properties of the production compoundR1 compared to a composite of compounds R-1587, 1589 and 1590 over atemperature range of 40°-100° C. The natural rubber/polybutadienecompounds of this invention exhibit low elastic modulus (E') and lossmodulus (E"). The dynamic response of a compared compound withoutpolybutadiene (R-1583) appears in FIG. 11.

The influence of polybutadiene on dynamic mechanical response isillustrated by a plot of tan delta versus temperature in FIGS. 12 and 13for compounds R-1, R-1583 and R-1587 and R-1, R-1589 and R-1590,respectively. As polybutadiene (BR) increases, the formation of amaximum occurs in the damping curve. The maximum is due to the lowerglass transition temperature of the polybutadiene (BR).

EXAMPLE 3

The natural rubber (NR) used in this example was SMR-5L. The bromobutylrubber, (BIIR) obtained from Polysar; Inc., is POLYSAR Bromobutyl X2having a viscosity, ASTM D1646 (ML-1+8'@125° C./257° F.) of 46, abromine content (wt. %) of 2.1 and a specific gravity of 0.93. Thepolyisobutylene (PIB) was obtained from Exxon Chemical Company. The PIB(Vistanex MM L-140) has a Flory viscosity average molecular weight of2.1×10⁶ according to "Vistonex Polyisobutylene Properties andApplications", Exxon Chemical Company, 1974. The ingredients listed inTable 3A were mixed in a Banbury mixer (Model BR) using a six minutemixing schedule (ASTM D15-72) to make elastomeric goods of thisinvention. The cure ingredients were added on a cooled 200×400 mm tworoll mill. Cure characteristics of the compounded stocks were determinedusing an oscillating disc rheometer. The compounds were molded and curedto 95% optimum cure at 150° C. or 170° C.

                                      TABLE 3A                                    __________________________________________________________________________                    R-1631                                                                            R-1634                                                                            R-1635                                                                            R-1636                                                                            R-1642                                                                            R-1644                                                                            R-1652                                                                            R-1653                                                                            R-1654                        __________________________________________________________________________    Natural Rubber  50  60  70  80  100 80  80  80  80                            Bromobutyl X2   50  40  30  20  --  20  20  20  20                            Polyisobutylene (L-140)                                                                       20  20  20  20  20  --  10  30  40                            N-765 carbon black                                                                            25  25  25  25  25  25  25  25  25                            Stearic acid    2   2   2   2   2   2   2   2   2                             Polymerized 1,2-dihydro-                                                                      2   2   2   2   2   2   2   2   2                             2,2,4-trimethylquinoline                                                      N--(1,3-dimethylbutyl)-                                                                       1   1   1   1   1   1   1   1   1                             N'--phenyl-p-phenylenediamine                                                 Aromatic Process oil                                                                          5   5   5   5   5   5   5   5   5                             (Dutrex 419)                                                                  Zinc oxide      5   5   5   5   5   5   5   5   5                             Sulfur          0.60                                                                              0.60                                                                              0.60                                                                              0.60                                                                              0.60                                                                              0.60                                                                              0.60                                                                              0.60                                                                              0.60                          N--oxydiethylenethiocarbamyl-                                                                 1.40                                                                              1.40                                                                              1.40                                                                              1.40                                                                              1.40                                                                              1.40                                                                              1.40                                                                              1.40                                                                              1.40                          N'--oxydiethylenesulfenamide                                                  N--oxydiethylene-2-benzo-                                                                     0.70                                                                              0.70                                                                              0.70                                                                              0.70                                                                              0.70                                                                              0.70                                                                              0.70                                                                              0.70                                                                              0.70                          thiazolesulfenamide                                                           __________________________________________________________________________

Sample sheets and compression set buttons were made as in Example 2.Fatigue measurements were made as in Example 2 except that the strainamplitude was either 175% or 200% at 5 Hz. Dynamic mechanicalproperties, tensile strength and elongation were determined and heataging done as in Example 2.

Hardness of the vulcanizates was determined according to ASTM D2240using a Shore A durometer. A ten second relaxation was allowed beforethe final reading.

Table 3B shows the results obtained from testing five compositions ofTable 3A. Each of these compositions contain the same amount of PIB(i.e., 20 phr of elastomer), but elastomer composition varies. It can beseen from Table 3B that replacing increasing amounts of natural rubberwith bromobutyl rubber results in compounds with increasing damping(increasing tan δ→ increasing damping) and improved fatigue life.

Table 3C shows the effect of the polyisobutylene content in acomposition whose elastomer is an 80/20 blend of naturalrubber/bromobutyl rubber. It can be seen that the optimal fatigue lifeis within the ranges taught earlier in the specification, i.e., 15-35.

                                      TABLE 3B                                    __________________________________________________________________________    EFFECTS OF BROMOBUTYL CONTENT ON THE PHYSICAL PROPERTIES                      OF BLENDS WITH NR SOFTENED WITH 20 PHR PIB                                    NR/BIIR   TENSILE                                                                              ELONGATION                                                                             TEAR STRENGTH                                                                            HARDNESS                                                                              COMP. SET                                                                            FATIGUE                   (PHR)     (MPa)  (%)      (kn/m)     (Shore A)                                                                             (%)    (kc)  TAN                 __________________________________________________________________________                                                              δ             R-1642                                                                            100/0 19.6   580      42.4       41      28     31    0.076               R-1636                                                                            80/20 16.8   595      38.9       41      32     63    0.135               R-1635                                                                            70/30 15.4   590      38.9       41      31     88    0.162               R-1634                                                                            60/40 16.0   625      30.6       40      29     88    0.181               R-1631                                                                            50/50 13.9   600      25.5       39      28     83    0.221               __________________________________________________________________________

                                      TABLE 3C                                    __________________________________________________________________________    EFFECTS OF POLYISOBUTYLENE LEVEL ON THE PHYSICAL                              PROPERTIES OF A NR/BIIR (80/20) BLEND                                         PIB      TENSILE                                                                             ELONGATION                                                                              TEAR STRENGTH                                                                            HARDNESS                                                                              COMP. SET   FATIGUE               (PHR)    (MPa) (%)       (kn/m)     (Shore A)                                                                             (%)    TAN δ                                                                        (kc)                  __________________________________________________________________________    R-1644                                                                             0   24.7  610       50.4       43      29     0.141                                                                              32                    R-1652                                                                            10   19.6  600       41.5       42      31     0.137                                                                              31                    R-1636                                                                            20   16.8  595       38.9       41      32     0.135                                                                              63                    R-1653                                                                            30   13.6  585       28.0       38      36     0.160                                                                              66                    R-1654                                                                            40   11.0  570       19.8       36      38     0.167                                                                              32                    __________________________________________________________________________

For purposes of the heretofore specification and hereinafter claims,"conventional sulfur vulcanization" refers to cure systems in which theratio of accelerator to sulfur is between about 0.2 and 0.5. When curedto optimum modulus by such systems, the sulfur crosslinks contain, onthe average, several sulfur atoms. Only a small number of the crosslinksare monosulfidic; more crosslinks are disulfidic and still more arepolysulfidic. The polysulfidic crosslinks may be of cyclic character."Efficient vulcanization", for such purposes, refers to a method ofcuring to reduce the number of sulfur atoms per crosslink formed, ascompared to conventional sulfur vulcanization. Efficient sulfurvulcanization may be obtained by (a) use of sulfur donor to replaceelemental sulfur completely or partially, (b) use of low sulfur, highaccelerator ratios in the curatives and (c) use of accelerator blendsand low sulfur. Semi effecient vulcanization, for such purposes, refersto cure systems that produce a cured elastomer intermediate in structureand thermal stability between those produced by effecient vulcanizationand conventional vulcanization. Semi-effecient vulcanization uses anaccelerator to sulfur ratio between about 0.6 and 2.5. The use of sulfurdonor to replace a part of elemental sulfur in conventionalvulcanization without altering the accelerator level is another way toprovide semi-efficient vulcanization.

Also, as used herein "isocyanate or blocked isocyanate" refers to acompound having two or more functional groups selected from isocyanateand blocked, but reactive, isocyanate functional groups.

What is claimed is:
 1. A soft, fatigue resistant vulcanizate adapted fortransmitting a load between moving mechanical parts comprising (a) 100parts by weight crosslinked elastomer, (b) about 10-75 parts by weightof substantially internally uncrosslinked, substantially linear polymerthat (i) is made from monomers consisting essentially of isobutylene,(ii) is a strain crystallizable elastic solid at 20° C. and (iii) has aviscosity average molecular weight (Flory) above about 1.3 million, (c)about 5-200 parts by weight particulate comprising carbon blackreinforcing particulate, said elastomer being crosslinked with (d)curative in an amount sufficient to crosslink said elastomer, whereinsaid polymer of (b) is dispersed throughout said elastomer of (a) in adiscrete microscopic phase.
 2. A vulcanizate in accordance with claim 1,wherein said curative comprises sulfur.
 3. A vulcanizate in accordancewith claim 1, wherein said substantially internally uncrosslinked,substantially linear polymer comprises a polyisobutylene having aviscosity average molecular weight (Flory) of about 1.8-2.4 million. 4.A vulcanizate in accordance with claim 1, 2 or 3, wherein said elastomercomprises natural or synthetic polyisoprene rubber.
 5. A vulcanizate inaccordance with claim 4, wherein said elastomer further comprisesbromobutyl rubber.
 6. A vulcanizate in accordance with claim 5, whereinsaid elastomer comprises said polyisoprene rubber and bromobutyl rubberin a weight ratio of between about 100:1 and about 1:1.
 7. A vulcanizatein accordance with claim 6, wherein said ratio is between about 7:3 andabout 1:1.
 8. A vulcanizate in accordance with claim 4, wherein saidpolymer of (b) is dispersed throughout said elastomer of (a) in anamount of about 15-35 parts by weight.
 9. A vulcanizate in accordancewith claim 1, wherein said particulate comprises 5-75 parts by weight ofsaid carbon black reinforcing particulate.
 10. A suspension bushinghaving a Shore A hardness below about 60, which comprises (a) 100 partsby weight crosslinked elastomer comprising natural or syntheticpolyisoprene rubber; (b) a dispersed phase in said elastomer comprisingabout 15-35 parts by weight of substantially internally uncrosslinked,substantially linear polyisobutylene that is a strain crystallizableelastic solid at 20° C. and has a viscosity average molecular weight(Flory) of about 1.8-2.4, (c) sulfur crosslinks, and (d) about 20-80parts by weight particulate filler consisting essentially of carbonblack reinforcing particulate wherein said dispersed phase has particlesof polymer of (b) below about 2 micrometers in diameter.
 11. Asuspension bushing in accordance with claim 10, wherein said rubbercomprises natural polyisoprene rubber.
 12. A suspension bushing inaccordance with claim 10 or 11, wherein said elastomer further comprisesbromobutyl rubber.
 13. A suspension bushing in accordance with claim 12,wherein said elastomer comprises said polyisoprene rubber and saidbromobutyl rubber in a weight ratio of between about 100:1 and about1:1.
 14. A suspension bushing in accordance with claim 13, wherein saidratio is between about 7:3 and 1:1.
 15. A suspension bushing inaccordance with claim 10 or 11, wherein said dispersed phase comprisespolyisobutylene having a viscosity average molecular weight (Flory)between about 2.0-2.2 million.